This invention relates generally to raster graphics systems and more particularly to apparatus for increasing the color and spatial resolutions of an existing system by reformatting the output transmitted to the system display device.
Raster graphics systems have in recent years become the preferred form of producing graphic images on a computer display device. Such systems generally comprise a host computer, a graphics controller, and a cathode ray tube (CRT) display device. In personal computer-based systems, the graphics controller is often contained on a plug-in card known as a graphics adapter. To generate graphics, display data from the host computer is passed to the graphics controller, which controls the data displayed. The display data is stored in bit planes within memory, with each bit in a plane corresponding to a picture element, or pixel, located on the display device screen. Several bit planes may be "stacked" one upon another so that each pixel location has as many associated bits as there are planes in the stack. For example, 4-bit planes provide a 4-bit pixel output that can produce up to 16 simultaneously displayable colors or 16 shades of gray on the system display device. The output from these planes typically forms the address to a number of internal registers whose data provides the color settings for the display. The group, or palette, of colors within the registers can be changed by a microprocessor (CPU) that controls the adapter. But the number of bits addressing the palette limits the number of colors that can be simultaneously displayed.
This limited color resolution is a major drawback of present raster graphics systems for personal computers. Color resolution may be defined as the number of pixel colors that can be simultaneously displayed and is a function of the bits per pixel output to the display device. For example, a system such as the IBM PC with a Color/Graphics Monitor Adapter (CGA) or Enhanced Graphics Adapter (EGA) produces four bits per pixel. This output can only provide 16 simultaneously displayable colors, with access to other palettes required for more colors. So few colors often do not produce a realistic image.
Another drawback of present personal computer-based raster graphics systems is their limited spatial resolution. Spatial resolution may be defined as the number of pixels that can be displayed on the display device screen and is given as horizontal and vertical quantities, i.e., 480 by 200. The greatest spatial resolution of the IBM PC with an EGA card is no better than 640 by 350 pixels. This spatial resolution is too low for high quality desk top publishing and high column spread sheets.
A third drawback inherent in present systems is the limited number of colors in the color palette. In the EGA, for example, a 4-bit output from the graphics controller addresses one of 16 palette registers internal to the EGA's circuitry. The content of each of these registers corresponds to a displayable color, with six bits from the register driving the display device. A 6-bit register can produce only 64 possible colors.